As is known in the art, electronic phase locked loops have a wide range of applications. One such application is in phased array systems. More particularly, transmit/receive elements in phased array antennas require stable phase and frequency reference provided to all the array elements in order to steer the beam in the desired direction. Several methods are known in the art for providing such a reference and accomplishing the required phase shift in each element. In multifunction systems, several such references must be provided simultaneously or alternatively upon demand, to various portions of the system. It is desirable to have a commonality of parts at the elements. It is also desirable to utilize a single system-wide frequency reference in lieu multiple interfering frequencies.
Frequency and phase reference may be provided to array elements in several ways. A first method provides an RF frequency reference and introduces a phase shift into the signal path. For transmit and receive this phase shift might be accomplished by switching high-pass and low-pass filters in and out of the RF signal path. The phase shifting may be accomplished in a “common-leg” circuit shared by the transmit and the receive paths.
Alternatively, referring to the receive case, one may use a mixer to down convert the RF signal within the element. This allows a system-to-element interface to be at a lower Intermediate Frequency (IF). The phase shifters may be placed in the LO path and therefore do not contribute to RF losses. Therefore several stages of RF gain may be omitted from the T/R element. An important application of the frequency conversion approach is when the Local Oscillator (LO) is produced on each array element such that the system interface can be at low frequency.
More particularly, the low frequency reference signal is used to synchronize the Local Oscillator by using a digital prescaler (divide by N) circuit as shown in FIG. 1. A portion of the tunable oscillator signal is divided by binary circuits until the frequency is equal to that of the injected reference signal. The phase of the reference signal is compared with the output of the prescaler. The oscillator is adjusted to make the phase difference zero. The phase shifter is shown, as know in the art, to be in the local oscillator (LO) path. The approach described above is not well suited to multifunction system or systems where the frequency at each element or group of elements must be varied independently. In the usual approach, a single reference frequency provided to all the elements precludes frequency agility since all the elements must remain locked to a particular multiple of this frequency reference.